The present invention relates generally to the use of laser beams for annealing purposes. More specifically, it relates to the use of laser beams to relieve inherent residual stress in very thin layers of ceramic materials, such as film-like thin layers of chemically vapor deposited (CVD) silicon nitride (Si.sub.3 N.sub.4) and silicon carbide (SiC) known respectively hereinafter as CVDSN and CVDSC. Structural ceramics are used in various systems relating to electronic, automotive, and military armor systems among others. Examples of substrate materials upon which the silicon nitride and silicon carbide may be chemically vapor deposited include graphite, mullite, and sintered silicon carbide.
Many studies have been previously conducted on laser interactions with various metals and semiconductors used in electrical circuit componetry. Examples of such latter studies include the following published articles:
Laser Induced Crystallization of Thin Semiconductor Films by H. P. Weber and W. Luthy, of the Institute of Applied Physics, University of Bern, Sidlerstrasse, 5, CH-3012 Bern, Switzerland, published on pg. 58-60 of the hardback book entitled "Proceedings of the International Conference on Lasers '79", by the STS Press of McLean, Va. 1980. Another such article on pgs. 775-783 in this same book is one entitled Laser Annealing of Semiconductors, by M. Bertolotti and G. Vitali, of the Institute of Fisica, University of Rome, in Italy.
The foregoing type of studies are of good interest but they relate to the specific non-analogous field of semiconductors, whereas it is believed that relatively little and only very limited study and research work has been done thus far in the field of laser processing of conventional ceramics and/or ceramic coatings. However, in this latter regard, one company so involved is United Technologies Research Center, in East Hartford, Conn. 06108, as evidenced by a recently published article entitled Observations of Laser Interactions with ceramics, by Francis S. Galasso and Richard Veltri. The brief article which duly acknowledges certain research center experiments by others and other bibliography contributions of interest, appears on pg. 253-254 in the February 1983 Issue (Vol. 52 No. 2) of the American Ceramic Society Bulletin. This article acknowledges among other things that attempts to heat ceramic bodies with a high intensity laser beam have usually resulted in crack formation and degradation. Notwithstanding this revelation, their studies have further revealed in one study regarding thermal barrier coatings on gas turbine engine components, that controlled crack formation by lasers in ceramics can be beneficial. However, their work involved the use of a high power nominally specified 6 KW CO.sub.2 laser specifically with respect to obtaining or generating cracking through a ZrO.sub.2 -Y.sub.2 O.sub.3 plasma sprayed coating on an apparently metal substrate, densifying the coating, alloying the coating with Al.sub.2 O.sub.3, or selective evaporation of dye particles from the coating.
Further background concerning the general subject matter involves work performed by the San Fernando Laboratory in Pacoima, Calif. Within their advertising brochures they offer services more particularly pertaining to a specialized procedure for effecting chemical vapor deposition. Their patented process is believed to be named Controlled Nucleation Thermochemical Deposition (CNTD). The observed brochure did not describe or discuss any contemplated laser annealing or other laser treatment thereof.
Another recent brief article published in the March 1983 issue of Lasers and Applications, pg. 33-35 is entitled Excimer Annealing Produces Efficient Solar Cells. It briefly describes obtaining greater efficiencies in silicon solar cells which have been annealed with excimer lasers, apparently by Helionetics, Inc. of Irvine, Calif. However, from the brevity of the discussion therein, it appears that the described laser annealing process is also non-analogous to the instant disclosure because it appears related to the more specific aspect of silicon semiconductor annealing to improve electrical properties rather than to ceramics and the relieving of mechanical stresses.
A further recent article published in the January/February 1983 issue of Optical Engineering, Vol. 22, pg. 117-120 is entitled Use of Laser Annealing to Achieve Low Loss in Corning 7059 Glass, ZnO, Si.sub.3 N.sub.4, Nb.sub.2 O.sub.5, and Ta.sub.2 O.sub.5 Optical Thin-Film Waveguides. It briefly describes obtaining lower optical loss in waveguides contemplated for thin film integrated optics. This was described as having been accomplished by annealing Si.sub.3 N.sub.4 thin films with CO.sub.2 laser irradiation. However it appears that their interest has been confined to its use in thin film integrated optics as exemplified by the publication of the article in Optical Engineering, wherein their goal is stated in the title to achieve low optical loss. This apparent intent is further strengthened by the absence of any reference to other uses of this laser annealing. This field has been quite separate from endeavors contemplating the use of CVDSN and CVDSC in structural applications such as in engines. The subject application is differently directed towards the use of CVDSN and CVDSC for wear, erosion, and corrosion protection to temperatures exceeding 1400.degree. C.